Conventional optical fiber amplifiers include active fibers having a core doped with a rare earth element. Pump power at a characteristic wavelength for the rare earth element, when injected into the active fiber, excites the ions of the rare earth element, leading to gain in the core for an information signal propagating along the fiber.
Rare earth elements used for doping typically include Erbium (Er), Neodymium (Nd), Yfterbium (Yb), Samarium (Sm), and Praseodymium (Pr). The particular rare earth element used is determined in accordance with the wavelength of the input signal light and the wavelength of the pump light. For example, Er ions would be used for input signal light having a wavelength of 1.55 .mu.m and for pump power having a wavelength of 1.48 .mu.m or 0.98 .mu.m; codoping with Er and Yb ions, further, allows different and broader pump wavelength bands to be used.
Traditional pump sources include single mode laser diodes and multimode broad area lasers coupled to the active fiber over single mode and multimode pumping fibers, respectively, to provide the pump power. Single mode lasers provide low pump power, typically in the order of 100 mW. Broad area lasers, on the other hand, provide high pump power, in the order of 500 mW. These lasers of high output power, however, cannot efficiently inject light signals into the small core of a single mode fiber. Consequently, the use of high power broad area lasers requires the use of multimode fibers for pumping optical amplifiers.
Broad area lasers generate multimode pump power and input the pump power to a non-active pumping fiber. This non-active pumping fiber in turn typically inputs the pump power through a coupler and into the inner cladding of a double-clad active fiber, acting as a multimode core for the pump pover.
In the amplifier fibers, pump power is guided into the inner multimode cladding of the fiber from which it is transferred into a single mode core doped with an active dopant.
A multimode fused fiber coupler has a theoretical coupling efficiency directly proportional to the ratio of the areas of the two fibers constituting the coupler. In an ideal case for two identical fibers, the coupling efficiency is approximately 50%. Typically, the coupling efficiency is in the range of 45-48%. This means that only about 45-48% of the total pump power output by the pump source into the pumping fiber actually passes from the pumping fiber into the inner cladding of the double-clad active fiber, while the remaining 52-55% remains in the pumping fiber.
Some systems use two optical fibers having different diameter of multimode cores to improve the coupling efficiency of the multimode coupler. However, such arrangements often lead to a waste of power due to the difficulty in matching the tapering of two cores of different size.
FIG. 1 is a block diagram of a conventional amplifying system containing a multimode pump source coupled to a primary fiber via a single traditional coupler. Primary fiber 1100 is a double-clad fiber. The information signal flows through its single mode core. Optical amplifier 1200, which may take the form of an Er/Yb doped double-clad active fiber, amplifies the information signal as it propagates through the single mode core of primary fiber 1100.
Multimode pump power generated by multimode pump 1300 is coupled into primary fiber 1100 via multimode pump fiber 1400 and coupler 1500. Coupler 1500 is a conventional fused fiber wavelength division multiplexer (WDM) type coupler. WDM couplers behave as multimode couplers for the pump power and transmit the single mode signal along the primary fiber substantially without coupling to the pump fiber. WDM couplers have maximum coupling efficiencies of 50% for the pump power, and typically have coupling efficiencies in the range of about 45%.
Multimode pump 1300 may take the form of a broad area laser that outputs multimode pump power of approximately 450-500 mW. Due to the coupling efficiency of coupler 1500, however, only about 45% of this pump power, or approximately 200-225 mW, enters primary fiber 1100. The remaining 55% of the pump power is lost, as it exits from pump fiber 1400.
Such a structure passes most of the outer modes of the pump power to primary fiber 1100, leaving the inner modes of the pump power to exit pump fiber 1400. Applicants have observed that the loss of pump power and the inefficient coupling of traditional couplers lead to insufficient coupling of the total pump power output from the multimode pump to the active fiber.
To increase the coupling efficiency, some conventional systems utilize microoptic couplers. Microoptic couplers couple optical beams using a wavelength selective mirror and a focusing lens. With this construction, microoptic couplers obtain much better coupling efficiencies than traditional WDM couplers, typically in the range of 89%. But microoptic couplers have several drawbacks which limit their use: (1) limited period of reliability because coupling of the optical beams takes place in air; (2) difficult to construct due to alignment difficulties; (3) very expensive compared to traditional WDM couplers; (4) unpredictable lifetime of the selective mirror dielectric layer due to the very high optical powers; and (5) high insertion losses.
Several articles in the patent and non-patent literature address multimode coupling techniques but do not discuss ways of overcoming the deficiencies of other conventional approaches described above. WO 96/20519 discloses a coupling arrangement for transferring light power between a multimode light source and a multimode optical fiber through a length of an intermediate feeding multimode optical fiber. The multimode feeding fiber has a progressively tapered portion and is fused to the multimode optical fiber at or near the tapered portion.
U.S. Pat. No. 4,877,305 discloses a mode mixer, or mode scrambler, achieved by inserting a length of fiber optic material inside a length of tubing, forming the tubing into a circular spiral having at least two coils, and then splaying the coils to be non-planar.
U.S. Pat. No. 4,676,594 discloses an optical fiber mode scrambler achieved by forming a deformation such as a groove or notch on one side of a multimode or graded-index optical fiber.
U.S. Pat. No. 4,974,930 discloses a mode scrambling arrangement for a multimode optical fiber that irradiates the cladding of the optical fiber using ultraviolet light to change the index of refraction of the cladding.
In Applicants' view, none of the known literature has recognized Applicants' discovery that conventional systems have failed to couple sufficient pump power, thereby leading to an inadequate overall pump power transfer efficiency.